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AT MICROFICHE REFERENCE LIBRARY A project of Volunteers in Asia The Design of Small Bore Sewer Systems by Richard Otis et. al. Published by: Technology Advisory Group World Bank 1818 H St. N.W. Washington D.C. 20433 USA Available from: same as above Reproduced by permission. Reproduction of this microfiche document in any form is subject to the same restrictions as those of the original document. TAG Technical Note No. 14 United Nations Development Programme Interregional Project INT,‘81/047 Executing Agency: World Bank The De&n of Small Bore Sewer Systems by Richard J. Otis and D. Duncan Mara, Technology Advisory Sror;p (TAG) A joint United Nations Development Programme and World Bank Contribution to the International Drinking Water Supply and Sanitation Decade UMllED n&liOns lml.r$lEm hP/Ul WP/cj2 IN/O1 TN/U2 TN/U3 ‘i’N/U4 TN/U5 I’N/Ub 'I'N/U7 ‘I’N /Utl i’N/US) TN/ IU Ttl/ll 'I'tu/ iL lJr/o2 LT’t‘T ‘J” Pl!Ht.T(1L\TTONS .ey I'HE IECHNULUGY AL)VLSUKY &KUUP \'l'AG) UNDP INTEKKEGIOhAL PKOJMX IN’l’/til/U47 A riodel for Lhe Development of a Self-help Water Supply Program; by Colin Glennie. Ventilated improved Pit Latrines: Recent Developments in Zimbabwe; bj Peter Morgan and 0. Duncan Ilara. Methods for Gathering Socio-cultural Data for Water Supply and Sanitation Projects; by Mayling Simpson-Hebert. PJ.anning of Communication Support (Information, Motivation and Education) in Sanitation Projects and Programs; by Hell Perrett. The Ventilated ,lmproved Double-Pit Latrine: A Construction Manual for botswana; by John van Nostrand and James G. Wilson. Pit Latrine Ventilation: Field Investigation Methodology; by Beverley KyZIl and lJ. Duncan Mara. Social Feasibility Analysis of Low-cost Sanitation Projects by heli Perrett. ; Ventilated lroproved Pit Latrines: Vent Pipe L)esikn Guidelines; by Bevt:riey Kyan and II. buncan Hara. Community-based Workshops for Evaluating and Planning Sanitation Programs: A Case Study ok Primary Schools Sanitation in Lesotho; by Piers Cross. Kurai Ventildted Lmproved Pit Latrines: A Field blanual for bOtSWaKia; by John van Nostrand and James G. Wilson. riandbook for Uistrict Sanitation coordinators; by Kebadire Basaako, Konald 0. Parker, Kobert B. Waller and James ti. Wilson. >lanual on the &sign, Construction and Maintenance of Low-cost Pour-flush Waterseal Latrines in Lndia; by A.K. Kay. klonltoring and Evaluation of Communlcacion Support Activities in Low-cost Sanitation Projects; by Heli E. Perrett. A Monitoring and Evaluation Manual for Low-cost Sanitation Programs in Lndia; by Ronald Parlato. l'h;! Uesign of Vcnti1atc.u Lmproved Pit Latrines; by 0. Duncan Mara. The Uesign of Small tiore Sewers; by Kichard Otis and D. Duncan Mara. Ventilated Lmproved Pit Latrines: Zimbabwean Brick lIesigns; by Peter Li. Morgan and Il. Duncan Mara. A Construct.Lon klanual for Urban Ventilated Lmproved Pit Latrines: Examples from Tanzania; by John van Nostrand, Daniel Makerere and Kobert Boydell. TAG lkchnkal Note No. 14 The Design of Small Bore Sewer Systems $ Ri;;t. &. and ?ichnology Ad,v&ory Group (TAG) d!p A joint United Nations Development Programme and World Bank Contribution to the International Drinking Water Supply and Sanitation Decade wmeo YATIOMS (I)**wW Copyright * 1985 The International Batik for Reconstruction and Development/TRR WORLD BANK 1818 H Street, NW Washington, DC 20433, U.S.A. All rights reserved Manufactured in the United States of America First printing May 1985 This Technical Note by Richard J. Otis and D. Duncan Mara is one of a series of informal Technical Notes prepared by T&r/ on various aspects of water supply and sanitation programs in developing countries. The initial emphasis of TAG was on the promotion of policy shifts from high-cost to low-cost on-site sanitation technologies. This emphasis is now being directed progressively to a focus on institutional development for on-site low-cost sanitation program delivery. The present note sets out provisional guidelines for the design of small bore sewers receiving pre-settled domestic wastewater. These guidelines are based on recent experience with small bore sewerage in Australia, Nigeria, the United States of America and Zambia; they have been written particularly for use in developing countries. Consequently, emphasis has been placed upon achieving simplicity of design consistent with reliability of operation. The note was originally prepared as an internal discussion document. Its wide distribution does not imply endorsement by the sector agencies,., government, or donor agencies concerned with programs, nor by the World Bank or the United Nations Development Programme. TAG will be interested in receiving comments and suggestions on the paper, and, in particular, information on costs of technology, delivery and support systems, and generally, information on experience in program implementation. All, communications should be addressed to the Project Manager, UNDP Project INT/81/047, Water Supply and Urban Development Department, The World Bank, 1818 H Street, NW. Washington, DC 20433. Richard N. Middleton Project Manager Acknowledgements The authors would like to express their gratitude to Mr. Cecil W. Rose (formerly of the Farmers Home Administration, United States Department of Agriculture) and Dr. M.G. McGarry (COWATER International, Ottawa, Canada) for their helpful comments on early drafts of this Technical Note. */ TPC: Technology Advisory Group established under the United Nations Development Programme, UNDP Interregional Project INT/81/047: Development and Implementation of Low-cost Sanitation Investment Projects (formerly Global Project GLO/78/006), executed by the World Bank. -_ -_ .--_-.-. -- . - --t (ii) TABLE OF CONTENTS Preface...............*. . . . . . . . . . . . . I. II. INTRODUCTION. ....................... System description .................. Component parts .................... Applicability in developing countries ......... DFsTGN CRITERIA ...................... Interceptor tank ................... Small bore sewers and appurtenances .......... Wastewater treatment options ............. III. CONSTRUCTION AND MAINTENANCE . . . . . . . . . . . . . . . . Materials ........................ Construction ..................... Maintenance ...................... Page (1) 1 1 2 4 7 7 11 17 17 17 19 22 IV. CASE STUDIES . . . . , . . . . . . . . . . . . . . . . . . . 23 Zambian sewered aqua-privies ............. 25 Australian sewered septic tanks ............ 27 Nigerian sewered aqua-privies ............. 28 American sewered septic tanks ............. 29 v. COSTS . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 ANNEXES I. Interceptor tank design ................ 43 II. Small bore sewer design examples ............ 47 (iii) Page Table1 : Table 2 : Figure1 : Figure 2 : Figure3 : Figure 4 : Figure 5 : Figure6 : Figure 7 : Figure 8 : Figure 9 : Figure10 : Figure 11 : Figure 12 : Figure 13 : Title Unit Cost for Small Bore Sewers. Conventional Sewers. Conventional Sewer and Water Main Installation from Selected Projects in the United States of America . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 Share of Total Project Cost by Component from 10 Uniform Grade Small Bore Sewer projects in the United States of America . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 Schematic diagram of a small bore sewer system........ (iv) Typical solids interceptor tank. [The tank may be buried by 300 mm or more to prevent unauthorized access by children or for garbage disposal]........... 6 A typical small bore sewer cleanout................... 14 Individual service connection lift station............ 15 Main in-line lift station with drop inlet............. 18 Sewered aqua-privy blocks in the Chipanda area of Matero, Lusaka, Zambia . . . . . . . . . . . . . . . . . . . . . . . . ..a....* 24 Sewered aqua-privy block in the Chipanda area of Matero, Lusaka, Zambia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 Sanitation block in New Bussa......................... 30 Sanitation block InNew Bussa......................... 31 External view of sanitation block in New Bussa, showing exposed connector sewer and junction boxes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 Site plan of the Mt. Andrew small bore sewer system . ..*......**m.............*.............m....... 33 Profile of the 50 mm diameter inflective gradient small bore sewer system at Mt. Andrew, Alabama........ 34 Site plan of the Westboro small bore sewerage systems . . . . . . . . . . . . . . . . . . ..*.......................... 36 (iv) c 0 .- System description I. INTRODUCTION 1.1 Small bore sewer systems (Figure 1) are designed to receive only the liquid portion of household wastewater for off-site treatment and disposal. Grit, grease and other troublesome solids which might cause obstruction in the sewers are separated from the waste flow in interceptor tanks installed upstream of every connection to the sewers; the solids which accumulate in the tanks are removed periodically for safe disposal. 1.2 Collecting only settled wastewater in this manner has four principal advantages: (a) (b) (cl (d) Reduced vater requirements. Since the sewers are not required to carry solids, large quantities of water are not needed for solids transport. Thus, unlike conventional sewers, small bore sewers can be employed without fear of blockages where domestic water consumption is low, where water-saving plumbing fixtures and appliances are widely used, or where long flat runs with few connections are necessary. Reduced excavation costs. With the troublesome solids removed, the sewers do not need to be designed to maintain a minimum flow velocity for self-cleansing. Therefore, rather than being installed on a straight path with a uniform gradient, they may be laid with curvilinear alignment with a variable or inflective gradient. This reduces excavation costs, since the sewer can follow the natural topography more closely than conventional sewers and avoid most obstructions within its path. Reduced materials costs. Peak flows which the small bore sewers must be designed to handle are lower than those experienced with conventional sewers because the interceptor tanks provide some surge storage which attentuates peak flows. Therefore, the sewer and any pumping equipment can be reduced in size (and pumps handling only liquids are simpler). In addition, expensive manholes can be replaced with uuch less costly cleanouts or flushing points, since mechanical cleaning equipment is not necessary to maintain the sewers in a free-flowing condition. Reduced treatment requirevents. Screening, grit removal and primary sedimentation or treatment in anaerobic ponds are not needed at the treatment works, since these unit processes are performed in the interceptor tanks. 1.3 Thus, small bore sewer systems provide an economical way to upgrade existing sanitation facilities to a level of service comparable to conventional sewers. Because of the lower costs of construction and maintenance and the ability to function with little water, small bore sewers can be used where conventional sewerage would be inappropriate. Small bore sewers therefore offer an opportunity of improving sanitation in areas which otherwise might not be upgraded. -2- 1.4 The principal disadvantage of the small bore sewer system is the need for periodic evacuation and disposal of solids from each interceptor tank in the system. Experience with the system is limited and mixed. Consequently, in spite of its obvious advantages it must be used judiciously and adopted only in situations where there is sufficient provision to ensure a strong organization for maintenance. This organization must also be able to exercise effective control over connections to the system. Special precautions should be taken to prevent illegal connections, since it is likely that interceptor tanks would not be installed in such connections, thereby introducing solids into a system which is not designed to handle solids. This could create serious operational problems. Component parts 1.5 Small bore sewer systems consist of: (a) house connections; (b) interceptor tanks; (c) the sewers and their appurtenances; and (d) a sewage treatment plant. Occasionally, individual pumping stations may be required to lift the effluent from the interceptor tank into the sewer to overcome adverse elevation differences; additionally, pumping stations may be required in the sewer system itself in very flat areas. (a) House connection. The house connection is made at the inlet to the interceptor tank. All household wastes, except for garbage and trash which must be removed for disposal else- where, enter the system at this point. Storm water must be excluded. (b) (4 Interceptor tank. The interceptor tank is a buried water- tight tank with baffled inlet and outlet. It is designed to detain the liquid flow for 12 to 24 hours and to remove both floating and settleable solids from the liquid stream. Ample volume is also provided for storage of the solids, which are periodically removed through an access port. Typically, a single-chamber septic tank is used as an interceptor tank. Severs. The sewers are small bore plastic pipe (minimum diameter of 100 mm) which are trenched into the ground at a depth sufficient to collect the settled wastewater from most connections by gravity. Unlike conventional sewers, small bore sewers are not necessarily laid on a uniform gradient with straight alignment between manholes or cleanouts. The sewer may have an inflective gradient; that is to say, the sewer may have dips so that sections of it remain full under static conditions. Also, the alignment may curve to avoid natural or manmade obstacles. The objective in the design and construction of small bore sewers is to utilize to the maximum extent the energy resulting from the difference in elevation between the upstream and downstream ends. -3- .! .\ \u/ 1 clcauou~s uuu muuho~.ie. Z~auouta and manhoies provide access to the sewers for inspection and maintenance. In most circumstances, cleanouts are preferable to manholes because they cost less and can be more tightly sealed to eliminate most infiltration and grit which commonly enter through the lids and walls of manholes* Also, they can be easily concealed to prevent tampering. They function as flushing points during sewer cleaning operations. (e) Vents. The sewers must be ventilated rg maintain free-flowing conditions. Vents within the household plumbing are sufficient, except where inflective gradient sewers are installed. In sue!; cases, the high points of the sewer should be ventilated either by locating the high points at connections or by installing a cleanout with a ventilated cap. (f) Lift stations. Lift stations are necessary where elevation differences do not permit gravity flow. Either residential or major lift stations may be used. Residential lift stations are small lift stations pumping wastes from the interceptor tank of one home or of a small cluster of homes to the sewer, while major lift stations are 1rJcated in the sewer line and service all connections withill a larger drainage basin. Whenever pumping becomes necessary, the difference in total annuitized costs between a small bore sewer system and conventional sewerage may be greatly reduced. Consequently, a close cost comparison would be required between the two before selection between them. Detailed design criteria for interceptor tanks and the hydraulic design of small bore sewers and sewer appurtenances are discussed in Section 2 below. Design examples are given in Annex II. 1.6 As will be seen from the system description above, the most important characteristic of small bore sewers is that they are designed to handle only the liquid portion of domestic wastes. Although the term "small bore sewers" has become commonly accepted, it is not in fact a very accurate description of the system, since the pipes need not be small diameter (the size being determined by hydraulic considerations and not constrained by other conditions), and the pipe system is not designed according to sanitary sewer practice. A more accurate description would be "solids-free sewers", but the best term is probably "effluent drains", as is used in the systems widely employed in Australia; this emphasizes the essential purpose of the sewers - to remove liquid effluents (from interceptor tanks) that cannot otherwise be disposed of on site - and so forms a natural link to the most likely application of small bore sewers in developing countries: to upgrade on-site disposal systems such as pour-flush latrines when changes in water use, housing densities or other conditions lead to difficulties in on-site effluent disposal. However, for consistency with other recent publications on this subject, in this note we have retained the term "small bore sewers". -4- Applicability In developing countries 1.7 Although on-site excreta and sullagei/ disposal systems are usually much less expensive in developing countries than off-site systems:/, there are situations when, due to adverse ground conditions (such as low soil permeability, shallow rock), on-site systems are technically infeasible. In such circumstances, off-site disposal is required and the full range of off-site disposal technologies has to be evaluated in technical, financial and economic terms. 'The available off-site disposal technologies are: id vault toilets and cartage; (b) conventional sewerage; and (4 small bore sewerage. 1.8 Vault toilets and cartage systems require a high degree of organizational capability within the institution (usually a municipality) responsible for operating the system: the vault-emptying equipment (commonly a vacuum tanker) has to arrive at each vault very close to the chosen emptying frequency (two to four weeks), otherwise the system fails; and the emptying equipment must be properly maintained. In many developing countries such a level of institutional competence is lacking and very often the system cannot be considered feasible for this reason. 1.9 Conventional sewer systems are so expensive that they are economically inappropriate in low-income communities. For example, studies by the World Bank (see footnote 2) showed that investment costs for conventional sewerage In eight major cities in developing countries ranged from USS600 to US$4,000 (at 1978 prices) per household, with corresponding annual economic costs?/ between USSl50 and USS650 per household. Such costs are clearly unaffordable when it is remembered that total annual household incomes are frequently less than US$500 and often below USS200. 1.10 Small bore sewer systems are especially suitable in developing countries in the following situations~/: (a) Sewered poul-flush toilet system. When the effluent from pour-flush toilets and household sullage cannot be disposed of on-site, small bore sewerage is almost always the best L/ Sullage (or "grey water") is all household wastewater other than the wastewater from toilets. 21 J.M. Kalbermatten, D.S. Julius and C.G. Gunnerson, Appropriate Sanitation Alternatives: A Technical and Economic Appraisal, Johns Hopkins University Press, 1982. 2/ These include annuitized investment costs and all operation and maintenance costs, including the cost of the water used for flushing the toilet. 51 The use of small bore sewers is not restricted to developing countries. As noted in Section IV, they have been successfully used in Australia and the United States, where they have shown considerable cost savings over conventional systems. -5- solution?/; it may be installed in new schemes, or it may represent the last stage in a planned sequence of incremental sanitation improvements?/. (b) Sewered septic tank system. When existing septic tank systems have failed, commonly due to the soil becoming unable to absorb increased wastewater flows resulting from improvements in the water supply distribution system or from increased housing densities, the septic tank effluent is best discharged into small bore sewers; this is almost always less expensive than abandoning the septic tanks and installing a conventional sewer network. In certain circumstances, especially in very flat terrain, it may be economically advantageous to install sewered septic tank systems - in conjunction with low-volume cistern-flush toilets - in new housing schemes as well. From the perspective of the householder there is little difference between small bore and conventional sewer systems, provided the interceptor tank is desludged regularlyI/. 1.11 In new schemes small bore sewerage often appears to have little advantage over conventional sewerage when compared to the latter in present worth terms. Yet the distribution of its costs between capital investment and operation and maintenance is quite different to those of conventional systems and is generally more appropriate to developing country conditions: capital costs - with their commonly high foreign exchange requirements - are lower; less skill is required in its construction; and its operation and maintenance are quite different to those of conventional systems and I! Sewered pour-flush toilet systems are best used in conjunction with at least a yard-tap level of water supply service. They may also be appropriate for use with a water supply system based on public standpipes; however, this depends on the standpipe density and the willingness and ability of the householders to carry sufficient water home. With sewered pour-flush toilets the total wastewater flow will normalPy be low, in the region of 30-80 litres per capita per day. / See J.M. Kalbermatten, D.S. Julius, C.G. Gunnerson and D.D. Mara, Appropriate Sanitation Alternatives: A Planning and Design Manual, Johns Hopkins University Press, 1982. A final in-house improvement might be the upgrading of the pour-flush toilet to a low-volume (three to five litres) cistern-flush toilet. LI Ideally, this should be the responsibility of the local water supply and sewerage agency, enabling the cost of desludging to be included as a normal part of the sewerage tariff, and so covered by regular small monthly payments rather than forming a large lump sum cost which householders may be unable or unwilling to afford. concrete cover,slab removable Inspection covers / . 5Omm 0 outlet ‘reinforced concrete base slab I m I Figure 2. Typical solids interceptor tank. [The tank may be buried by 300 mu or more to prevent unauthorized access by children or for garbage disposal.]